The present invention relates to the drilling of oil and gas wells and in particular, to an apparatus useful in drilling, completing and working over offshore wells.
The more accessible areas of the world that may contain hydrocarbon deposits have been explored and the search now extends out into deeper waters, i.e. those more than 1500 feet deep. The production of hydrocarbon deposits from these deeper waters requires specially designed production facilities and platforms. The most logical choice for a platform is what is referred to as a tension leg platform since it is more economical to build than a conventional bottom-supported platform. The economics result from the elimination of the steel structure that is required for supporting a production platform on the surface from the bottom in extremely deep waters.
If a tension leg platform is to be used for the production platform, it is more convenient and economical if the platform is also used for drilling the wells during the development of the production reservoir. While the platform can be used for drilling the production wells, it must incorporate features that were not required in bottom-supported platforms used in more shallow waters. The features required are those that would make provisions for riding out storm conditions which will cause the platform to move laterally on the surface of the water. In a conventional bottom-supported platform, the platform does not move laterally with respect to the subsurface wells in storm conditions. This reduces and in fact eliminates any lateral stress on the wellhead or production tubing coupling the platform to the wellhead. In the case of a tension leg platform since the platform freely floats on the surface, storm conditions can cause considerable lateral movement which will exert stresses onto the subsea well equipment which are not present in prior systems.
The stresses exerted on the wellhead equipment and foundation during storm (high offset) conditions are particularly high during drilling operations when the equipment extending from the platform to the wellheads exerts high bending, axial and laterial loads to the marine wellhead system. In conventional drilling operations the subsea equipment would include a blowout preventer assembly (BOP) plus a lower marine riser package (LMRP) assembly all of which would be connected to the surface through the marine riser. The drill string would then extend through the riser and the subsea wellhead equipment. The total height of the blowout preventers plus the marine riser package exceeds the normal height of wellhead equipment. In addition, the total weight of the equipment is considerably more than a conventional production wellhead. Likewise, the marine riser is considerably larger than normal production tubing and thus weighs more and exerts more force on the wellhead equipment if the platform moves laterally under storm conditions. All of these factors result in a considerably increased lateral load on the subsea wellhead during storm conditions.
An obvious means for reducing the load on the subsea wellhead during storm conditions is to remove the equipment from the wellhead that is tied-back and tensioned from the surface vessel to reduce the load on the wellhead. It is possible to remove the LMRP assembly and the riser and drill string from the wellhead assembly leaving only the blowout preventers which can be closed to safely secure the subsea well. Once the marine riser and the LMRP assembly is removed from the wellhead it will present a problem since it must either be raised to the surface or it must be constrained to ensure that it does not cause damage to the tension legs or well production risers of the platform or to other apparatus on the floor of the ocean. It can be readily appreciated that the long marine riser with the LMRP assembly attached to its lower end extending down to near the ocean floor provides an object which will swing with considerable force as storm conditions laterally move the platform.
Normally, it would be desirable to bring the riser and LMRP to the surface. However, it requires considerable time to disassemble the individual sections of the riser pipe and store them on the platform. Further, storage facilities for the riser pipe may not be normally present or quickly and easily accessible. Thus, it may be preferable that means be provided for securing the riser pipe and LMRP assembly to the ocean floor so that storm conditions will not cause it to move or swing around and cause damage to either the tension legs of the platform or to subsea equipment.